Bifocal spectacle lenses have been known for hundreds of years. In such lenses a first region of the lens is typically provided with a first focal length while a second region of the lens is provided with a second focal length. The user looks through the appropriate portion of the lens for viewing near or far objects.
In recent years as the popularity of contact lenses has grown, there has been an increased interest in multifocal contact lenses. Multifocal contact lenses utilizing an approach similar to that used in spectacle lenses are described in Contact Lenses: A Textbook for Practitioner and Student, Second Edition, Vol. 2 on pages 571 through 591. Such lenses have serious drawbacks, however, because they require that the lens shift on the eye so that different portions of the lens cover the pupil for distant and close vision. This design is disadvantageous because it is difficult to insure that the lens will shift properly on the eye for the desired range of vision.
In another design for a bifocal contact lens described in the above-referenced textbook, a central zone of the lens is provided with a first focal length and the region surrounding the central zone is provided with a second focal length. This design eliminates the necessity for shifting the lens by utilizing the phenomenon of simultaneous vision. Simultaneous vision makes use of the fact that light passing through the central zone will form an image at a first distance from the lens and light passing through the outer zone will form an image at a second distance from the lens. Only one of these image locations will fall on the retina and produce a properly-focused image, while the other image location will be either in front of or behind the retina. The human eye and brain will, to a great extent, work together to ignore the improperly-focused image. Thus the user of such a lens receives the subjective impression of a single, well-focused image. A disadvantage of such a lens is that, if the central zone is made large enough to provide sufficient illumination in its associated image location in low light situations, i.e., when the patient's pupil is dilated, the central zone will occupy all or most of the pupil area when the pupil contracts in a bright light situation. Thus bifocal operation is lost in bright light. Conversely if the central zone is made small enough to provide bifocal operation in bright light situations, an inadequate amount of light will be directed to the image location associated with the central zone in low light environments. Because the central zone is commonly used to provide distant vision, this can create a dangerous situation when the user of such a lens requires distant vision in low light situations such as when the user must drive a motor vehicle at night.
U.S. Pat. Nos. 4,210,391; 4,340,283; and 4,338,005, all issued to Cohen, teach the use of a plurality of annular regions that direct light to multiple foci and rely upon simultaneous vision to discard unfocused images. The annular zones are designed to provide the lens with a diffractive power. Typically a first focal length will be associated with a zero order diffracted light and a second focal length will be associated with the first order diffracted light.
In a typical prior art contact lens having diffractive power the various zones are separated by steps having an optical height equal to .lambda./2 where .lambda. is the design wavelength of light. The optical height is defined as h/(n.sub.1 -n.sub.2) where h is the physical height, n.sub.1 is the index of refraction of the lens and n.sub.2 is the index of refraction of the surrounding medium, typically the tear fluid. The steps are usually made as sharp as possible in order to provide efficient diffraction. The use of such sharp steps, however, has two significant disadvantages. The first disadvantage is that sharp outer steps have been known to cause corneal damage known as staining and also scarring. The second is that deposits from the tear fluid components tend to accumulate in the inner corners of the steps. Furthermore such deposits are difficult to remove from the lens when the inner corner is sharp.